Cryptology ePrint Archive: Report 2014/236

Linear Sequential Circuit Approximation of Acterbahn Stream Cipher

Shazia Afreen

Abstract: Achterbahn stream cipher is proposed as a candidate for ECRYPT eSTREAM project which deals with key of length 80-bit. The linear distinguishing attack,which aims at distinguishing the keystream from purely random keystream,is employed to Achterbahn stream cipher. A linear distinguishing attack is based on linear sequential circuit approximation technique which distinguishes statistical bias in the keystream. In order to build the distinguisher, linear approximations of both non-linear feedback shift register (NLFSR) and the non-linear Boolean combining function R:F_2^8&#8594;F_2 are used. The keystream sequence generated by this algorithm consist a distinguisher with its probability bias&#12310; 2&#12311;^(-1809). Thus, to distinguish the Achterbahn, we only need 1/&#949;^2 =&#12310;&#12310;(2&#12311;^1809)&#12311;^2=2^3618 keystream bits and the time complexity is about 10/&#949;^2 =2^3621.3 which is much higher than the exhaustive key search O(2^80).